Good question. I think owners pay property taxes in most places (the only exception I can think of is Israel), and they certainly do in NY. But I would imagine that in the annual ritual of deciding what the allowed rent increases are, property tax increases would be taken into account.

Maybe property taxes on rent-controlled buildings simply doesn't go up much? After all, the "value" (in the normal, vague sense, not the fashion of using it as a synonym for price) of a rent-controlled building, in the sense of the income stream it provides, doesn't go up much, whatever happens to its price. So maybe the law has two ways of determining value, either based on purchase price, or based on rental value.

There was (is?) something similar with faculty housing. NYU attracts potential faculty with inexpensive housing. Back in the 80s, tax reform meant that the difference between the rent and market value (a lot...) would become taxable income. But their lobbyists managed to insert into the law an alternative way of calculating the size of the subsidy, namely the difference between the university price and similar, non-subsidied apartments in the same building. They just happened to have a few rent-controlled tenants from before they bought the building that they hadn't managed to evict.....